generalize fct_sumE

[t6s]

Motivation for this change

This PR generalizes lemma fct_sumE from a pointwise equality

$$(\sum_k f_k)(x) = \sum_k f_k(x)$$

to the corresponding functional equality

$$\sum_k f_k = \lambda x. \sum_k f_k(x),$$

which should be more easily usable.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

- [ ] added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels